The present invention relates to a sol-gel process for the production of manufactured articles containing an incompressible index and to the manufactured articles so produced.
In particular, the present invention relates to a process for the production of preforms for optical fibers and to the preforms so a produced.
As is known, optical fibers comprise at least one central port and a covering part mode from glass materials with different refractive indices. The difference in refractive index between the two parts of the fiber and the almost glancing angle with which the light radiation impacts on the interface between the two parts of the fiber determine a condition of total reflection, therefore confining the light radiation to the central part. This difference in refractive index is normally achieved by a different chemical composition in the two parts of the fiber, and generally the material with higher index of refraction is in the central part. The materials more commonly employed for the production of optical fibers are glass of mixed silicon dioxide/germanium oxide composition for the central part of the fiber and high-purity silicon dioxide for the covering.
The optical fibers are produced by spinning the so called xe2x80x9cpreformsxe2x80x9d that consist of two coaxial cylinders, a central core and an external covering, corresponding respectively to the central part and to the covering of the final optical fiber. Typical dimensions of the preforms vary between about 0.5 and 1 meter in length, with diameters varying between about 5 and 20 centimeters. The diameter of the core is generally about a third of the overall diameter of the preform. During the spinning process, the preform is heated to a temperature lower than the melting point of the vitreous oxides that compose it, but sufficient to cause them to soften. A material is thus obtained with enough viscosity to maintain the geometric relationship of the parts that compose the preform, but sufficiently low to allow the formation of the fiber by traction.
Traditionally, production of preforms for optical fibers starts from a vitreous core already of final dimension and density, obtained for example by the normal technique of melting and subsequent solidification of oxides. The material of the covering is subsequently deposited onto the core, generally employing the technique of chemical deposition from vapor phase, known in the art as xe2x80x9cChemical Vapor Depositionxe2x80x9d or CVD, which consists of making two or more either gaseous reagents or reagents in vapor-phase to react at suitable temperature; the reaction product is the material desired. In the case of the optical fibers, silicon tetrachloride (SiCl4) and oxygen are generally employed, giving the reaction:
SiCl4+O2xe2x86x92SiO2+2Cl2xe2x80x83xe2x80x83(I)
The silicon dioxide (SiO2) so formed is deposited on the core that is present in the reaction chamber. This covering of SiO2 is initially porous and is densified by subsequent heat treatment.
This technique, used for a long time in the preparation of preforms, has the disadvantage that the SiO2 deposition phase for CVD requires very long times; typically, it requires about 7 hours to give a 2 cm covering thickness after densification.
Alternative techniques to the CVD have been assessed to overcome the problem. In particular, the use of the sol-gel technique, which gives vitreous materials by starting from generally hydroalcoholic solutions, has been much studied.
The name sol-gel generically defines a wide variety of processes that differ in procedural detail or choice of reagents. All sol-gel processes share the following phases:
hydrolysis in a hydroalcoholic solution, called sol, of a MXn compound, indicated generically as the precursor, containing the M cation, which is at least trivalent and preferably tetravalent, the vitreous oxide of which needs to be formed. The hydrolysis leads to the formation of M-OH groups;
polycondensation of the M-OH groups according to the reaction;
M-OH+M-OHxe2x86x92M-O-M+H2Oxe2x80x83xe2x80x83(II)
with the formation of an oxide polymer, called gel, that occupies all the volume initially occupied by the solution. This phase is generally defined as gelling;
drying of the gel giving a monolithic dry and porous body, with apparent density (weight divided by the geometric volume of the monolithic body) within the range of about {fraction (1/12)} and ⅕ of the theoretical density of the corresponding non-porous oxide. The drying could be achieved by controlled evaporation of the solvent, giving a body known in the art as xe2x80x9cxerogelxe2x80x9d, or by hypercritical extraction of the solvent, giving an xe2x80x9caerogelxe2x80x9d;
possible densification of the dry gel by heat treatment, giving a vitreous body of theoretical density.
The sol-gel technique shows promise for the production of preforms for optical fibers because it is relatively low-cost, the production times are almost independent of the dimensions of the vitreous body to be produced, and it gives good control of the chemical composition and the dimensions of the final vitreous body.
This technique is already used for the production of the core which is a solid cylinder of a homogeneous glass of mixed silicon dioxide and germanium oxide composition, that is obtained with extreme simplicity by this method.
The covering, consisting of a hollow cylinder, can be produced easily by the sol-gel method, by inserting sol into a cylindrical container to a volume of less than the volume of the same container and setting the container in rapid rotation on its axis for all the time required for gelling, so that the sol is made to adhere to the cylindrical wall of the container by centrifugal force. The gel so obtained presents a cylindrical external surface corresponding to the inside surface of the container and an internal cylindrical surface corresponding to the free equilibrium surface of the sol itself under the action of the centrifugal force. The production of vitreous tubular bodies in this way is described, for example, in U.S. Pat. No. 4,680,045.
U.S. Pat. No. 4,775,401 describes a process for the production of a preform of optical fiber whose covering is produced by sol-gel and then made denser around a core produced apart.
Even though it is possible to produce the core and the covering separately by sol-gel, a sol-gel process that produces a complete preform is desirable. In fact, the formation of a preform starting from two separate bodies creates some problems, like, for example, the possibility that polluting particles or air bubbles will be trapped between the two parts during the phase of densification to give the preform. These defects are retained in the final optics fiber and constitute sources of diffusion of light with consequent loss of efficiency in the transmission. Furthermore, the movement of two separate parts during the phases of drying and densification is more difficult than would be the case if a preform consisted of solidly integrated parts, as happens in the case of the deposition of the covering on the core by CVD.
Until now, however, it has not been possible to produce a similar preform by depositing the covering by sol-gel onto a core already at final density; this is because during the gelling phase a phenomenon to known as syneresis occurs, by which a gel in formation decreases its volume of about 1-3% compared to the volume of the sol, with a isotropic contraction toward its center. If the gel contains inside it an incompressible body, like a dense core of preform, contraction is prevented in the radial direction, but occurs tangentially, giving rise to intense lateral traction forces that leads to the destruction of the gel.
U.S. Pat. No. 4,786,302 describes a process for the production of all the components of the preform that avoids the problem of gelling against a rigid body that opposes the syneresis. According to this process, a hollow cylinder gel of a first composition is prepared by centrifugation, according to the process of U.S. Pat. No. 4,680,045 cited before. A sol with a second composition, different from the first, is poured into the hollow so produced, and allowed to gel. In this way, two concentric wet gels are obtained, that are then dried together and could be densified together to produce the preform. With this process however the two phases of gelling (and respective syneresis) of the different sections of the preform happen in successive moments. In particular, the syneresis of the covering (whose internal diameter reaches the dimensions for which the wet gel is stable) occurs first; subsequently the second sol is inserted in the hollow, that initially occupies the volume defined by the internal diameter of the external gel, but following the syneresis gives rise to a gel of slightly smaller diameter. The result is two physically separated concentric bodies, so possible problems due to the presence of impurities or air bubbles previously discussed are not resolved.
It is therefore impossible, with the present state of the art, to produce a preform for optical fiber in which the covering is produced by sol-gel directly on the core and integral with It.
The object of the present invention is to provide a sol-gel process for the production of manufactured articles containing an incompressible insert, as well as to provide manufactured articles obtained with the process, in particular preforms for optical fibers.
This object is achieved according to the present invention by a sol-gel process for the production of manufactured articles containing an incompressible insert, comprising the steps of:
a) providing an incompressible insert;
b) providing a container which can retain said incompressible insert in rigidly fixed position to define a space between the inside surface of said container and the external surface of said insert, and which can be rotated around the axis of said insert;
c) fixing said insert to the inside of said container in such a way as to rotate said insert as one with said container;
d) filling said space with a sol;
e) setting said container containing said sol and said insert in rotation around the axis of the latter for all the necessary time to the complete gelling of said sol;
f) opening said container and extracting the composite comprising a wet gel adhering to said incompressible insert;
g) drying said wet gel.
The inventors have surprisingly found that, contrary to what was previously known, it is possible to produce a body for sol-gel around an incompressible insert without the syneresis causing the destruction of the gel.
In particular, it has been found that the best results are obtained using an insert preferably of regular polygonal section. Even more preferably, the insert is of circular section, i.e. it is substantially cylindrical.
Furthermore, it has been found that the best results are obtained if, during all the time necessary to the gelling, the sol is rotated around the axis of symmetry of the insert at a predetermined speed depending on the radius of the same insert in the case in which it has a circular section or, in the case in which the insert is a regular polygon in section, depending on the radius of the circumferences inscribed and circumscribed on the section of the polygon. The reasons are not known, but it is thought that In these conditions the syneresis could be counterbalanced by the centrifugal force that acts on the sol.
The container is preferably placed in rotation with such speed for which P, the product of the angular velocity to measured in radians per second (rad/s) and the radius r of the insert measured in centimeters (cm) is between about 20 and about 250 radxc3x97cm/s.
Preferably, and particularly in the case of obtaining the preform for optical fibers, the drying of the gel is carried out by the hypercritical way. In this case, the last operations of the process include the following steps:
opening of the container and extracting the composite constituted by the wet gel and the incompressible insert, inside a bath containing a liquid, in such a way that the surface of the wet gel is not ever exposed to the air;
inserting the bath containing the liquid and the composite wet-gel/incompressible-insert in an autoclave;
extracting the solvent in hypercritical conditions giving a manufactured article constituted by the dry gel containing the incompressible insert.
The original liquid with which the gel is covered in the container, and that is contained in the same pores of the gel, is an aqueous liquid containing alcoholic residues that could vary according to the precursor used in the sol-gel synthesis. For instance, ethanol is the common alcoholic residual left by the hydrolysis of the tetra-ethyl-ortho-silicate (TEOS), one of the most widely used alkoxides in sol-gel synthesis.
In particular, the liquid both in the bath in which the gel is immersed, and inside the pores of the same gel, is substituted by a liquid suitable for hypercritical extraction.
Finally, the processes described above could be completed by a final operation, the densification of the dry gel around the incompressible insert by means of a suitable heat treatment.